From what i've read it was because of in the past when medicine and hygene weren't as good as they we're now so if a baby lived past 100 days people believed that meant they would make it to see their first birthday. The number 100 also has a meaning of maturity , signifying a baby passes through a perfection period safely into a human being. It is also a big day in relationships in korea which i think ties into if they reach a 100 days that means they will last? That's my opinion based on what i've read . ^^ Hope that helps :]